Diesel engine technology is widely known as an example of an engine technology where the fuel is injected into its cylinders, which is so called "in-cylinder injection engine" or "direct injection engine". For spark ignition (gasoline) engine also, in-cylinder injection type has recently been proposed. For such in-cylinder injection engines, it is required that the fuel pressure pulsation should be small enough to achieve stable injection as well as the fuel injection pressure should be sufficiently high.
Therefore, a single-cylinder high-pressure fuel pump which is simple in structure, produced at a low cost and compact is already known.
Since the single-cylinder high-pressure fuel pump has only one plunger, it generates a larger pulsation width in the fuel pressure than a multi-cylinder high-pressure fuel pump does. Therefore, a metal bellows type or metal diaphragm type pulsation absorber is provided in a fuel supply system to absorb the pulsation.
FIG. 8 is a diagram showing the configuration of a fuel supply system for a direct injection gasoline engine disclosed by Japanese Laid-open Patent Application No. 9-310661, for example. In this fuel supply system for a direct injection gasoline engine, the pressure of fuel (gasoline) stored in a fuel tank 70 is increased to a low level by a low-pressure fuel pump 71 and then the fuel is supplied to a high-pressure fuel pump 73 by a low-pressure pipe 72. The high-pressure fuel pump 73 further increases the pressure of the fuel to a high level by the reciprocating motion of a plunger 75 driven by the cam shaft 74 of an unshown engine and discharges the fuel from an outlet port 76. This outlet port 76 is connected to a common rail 79 through a high-pressure check valve 77 and a high-pressure pipe 78. High-pressure fuel stored in the common rail 79 is supplied to injectors 81 attached to the respective cylinders 80 of the engine through branch passages 82.
This common rail 79 is connected to a metal bellows type pulsation absorber 85. This metal bellows type pulsation absorber 85 is constituted such that a barrel portion is composed of metal bellows 85a, an opening at one end of the metal bellows 85a is closed by an end plate 85b, a peripheral portion at the other end of the metal bellows 85a is connected to the end surface 85c of the absorber by welding or the like, a closed space is formed inside the metal bellows 85a, and gas such as nitrogen or argon is charged into this closed space. The pressure pulsation of high-pressure fuel to be applied to the end plate 85b is absorbed by the expansion and contraction of the metal bellows 85a so that the pressure pulsation of the high-pressure fuel supplied into the common rail 79 is absorbed.
FIG. 9 is a sectional view showing the configuration of a high-pressure fuel supply system 10D equipped with a metal diaphragm type pulsation absorber. The high-pressure fuel supply system 10D comprises a high-pressure fuel pump 11, a low-pressure damper 14 provided in an inlet passage 12 connected to an inlet port side of the high-pressure fuel pump 11 and equipped with metal bellows 14a, a high-pressure damper 90 provided in an outlet passage 15 connected to an outlet port side of the high-pressure fuel pump 11 and equipped with a metal diaphragm 90m, and a high-pressure check valve 17 arranged on a downstream side of the high-pressure damper 90, all of which are integrally arranged in a casing 100.
The high-pressure pump 11 pressurizes the low pressure fuel supplied from the unshown fuel inlet port through the inlet passage to a high pressure level and discharges it to the outlet passage 15 by utilizing the plunger 112 which is arranged in a cylinder 111 in such a manner it can reciprocate and is driven by a cam 19 whose rotational speed is a half of an unshown engine's crank speed.
The metal diaphragm type pulsation absorber 90 is provided to suppress the pressure pulsation of this discharged high-pressure fuel. As shown in FIG. 9 and FIG. 10, the metal diaphragm type pulsation absorber 90 comprises a case 91 constituting one part of a high-pressure container, a plate 92 constituting the other part of the high-pressure container, and a flexible thin metal disk-like diaphragm 90m forming a first high-pressure chamber 93 with the above case 91 and a second high-pressure chamber 94 with the above plate 92. The above second high pressure chamber 94 is connected via multiple through holes 96 with a recess 95 which constitutes a path between the first passage 15P to an outlet of the high-pressure fuel pump located in the casing 100 and the second passage 15Q to a check valve 17. The above first high-pressure chamber 93 is filled with unshown gas from a gas filling port 97 formed in the case 91 at a predetermined pressure. This predetermined pressure is required to absorb the pulsation of the high-pressure fuel running through the second passage portion 15Q from the first passage portion 15P through the recessed portion 95.
When pulsation occurs in the above fuel while the first high-pressure chamber 93 is filled with gas and the second high-pressure chamber 94 is filled with fuel, the diaphragm 90m absorbs the pressure pulsation by bending towards the case 91 and towards the plate 92 from the balance point (for example, a position having no deflection shown by a bold line in FIG. 10) where the total of the gas pressure in the first high-pressure chamber 93 and the spring force of the diaphragm 90m itself becomes equivalent to the average pressure of the fuel.
However, in the metal diaphragm type pulsation absorber 90, since the metal diaphragm which is an expansion member expands and contracts repeatedly by an amount equivalent to the pressure pulsation of fuel with the balance point at an average fuel pressure as a center, when this fuel supply system for a direct injection gasoline engine is used in a fuel pressure variable system, the balance point changes, whereby average stress generated in the diaphragm alters, thereby causing a problem with durability.
For instance, when the variable range of fuel supply pressure of the fuel supply system is 5 to 10 MPa and the balance point of the metal diaphragm 90m is set to P.sub.0 =7.5 MPa which is the center of the above variable range, as shown in FIG. 10, if P.sub.0 =10 MPa, the metal diaphragm 90m vibrates with the balance point greatly displaced to the first high-pressure chamber 93 side and if P.sub.0 =5 MPa, the metal diaphragm 90m vibrates with the balance point greatly displaced to the second high-pressure chamber 94 side. Since average stress applied to the metal diaphragm 90m becomes larger as the balance point displaces more from the center of the variable range, the durability of the metal diaphragm 90m deteriorates.
To prevent deterioration in the durability of the metal diaphragm, it is conceivable, for example, to reduce the volume of the first high-pressure chamber 93 so as to lessen the amount of charged gas. In this case, pulsation absorption capability becomes less. It is also possible to improve the durability of the metal diaphragm by reducing average stress to be applied to the metal diaphragm by increasing the diameter. However, in this case, the pulsation absorber becomes large in size.
Even when a metal bellows type pulsation absorber is used as a high-pressure damper, if fuel supply pressure is made variable, the gas charging pressure must be reduced to achieve the minimum fuel pressure and the number of pleats of the metal bellows must be increased to obtain the large expansion width of the metal bellows with the result that the system becomes large in size.